Direct carbon-13 and C13-proton nuclear magnetic resonance (nmr) provides a convenient and facile method for studying the biosynthetic origin of complex natural products. Extension of methods previously developed will provide insight into the nature of the intermediates involved in the formation of complex natural products. NMR of metabolites derived from easily available C13-enriched substrates, coupled with the new and useful techniques of continuous wave and proton noise decoupling and mass spectrometry allows quantitative and qualitative determination of isotope enrichment at specific carbon atoms without recourse to extensive chemical degradation. Among the natural products to be studied are the macrolide antibiotics magnamycin and spiramycin; the fungal antiviral agents rifamycin B and mitomycins. The nature of the biosynthetic intermediates involved in the formation of such terpenes as helicobasidin, illudin S and hirsutic will be studied. These spectrometric methods for biosynthesis study will also be applied to naturally occurring materials of botanical as well as microbial origin.